[PATCH v3 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

[Lance Yang <ioworker0@gmail.com>]

When the user no longer requires the pages, they would use
madvise(MADV_FREE) to mark the pages as lazy free. Subsequently, they
typically would not re-write to that memory again.

During memory reclaim, if we detect that the large folio and its PMD are
both still marked as clean and there are no unexpected references
(such as GUP), so we can just discard the memory lazily, improving the
efficiency of memory reclamation in this case.

On an Intel i5 CPU, reclaiming 1GiB of lazyfree THPs using
mem_cgroup_force_empty() results in the following runtimes in seconds
(shorter is better):

--------------------------------------------
|     Old       |      New       |  Change  |
--------------------------------------------
|   0.683426    |    0.049197    |  -92.80% |
--------------------------------------------

Suggested-by: Zi Yan <ziy@nvidia.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
---
 include/linux/huge_mm.h |  2 ++
 mm/huge_memory.c        | 75 +++++++++++++++++++++++++++++++++++++++++
 mm/rmap.c               |  3 ++
 3 files changed, 80 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2daadfcc6776..fd330f72b4f3 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -38,6 +38,8 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		    unsigned long cp_flags);
 void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
 			   pmd_t *pmd, bool freeze, struct folio *folio);
+bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
+			   pmd_t *pmdp, struct folio *folio);
 
 vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
 vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 145505a1dd05..d35d526ed48f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2690,6 +2690,81 @@ static void unmap_folio(struct folio *folio)
 	try_to_unmap_flush();
 }
 
+static bool __discard_trans_pmd_locked(struct vm_area_struct *vma,
+				       unsigned long addr, pmd_t *pmdp,
+				       struct folio *folio)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	int ref_count, map_count;
+	pmd_t orig_pmd = *pmdp;
+	struct mmu_gather tlb;
+	struct page *page;
+
+	if (pmd_dirty(orig_pmd) || folio_test_dirty(folio))
+		return false;
+	if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
+		return false;
+
+	page = pmd_page(orig_pmd);
+	if (unlikely(page_folio(page) != folio))
+		return false;
+
+	tlb_gather_mmu(&tlb, mm);
+	orig_pmd = pmdp_huge_get_and_clear(mm, addr, pmdp);
+	tlb_remove_pmd_tlb_entry(&tlb, pmdp, addr);
+
+	/*
+	 * Syncing against concurrent GUP-fast:
+	 * - clear PMD; barrier; read refcount
+	 * - inc refcount; barrier; read PMD
+	 */
+	smp_mb();
+
+	ref_count = folio_ref_count(folio);
+	map_count = folio_mapcount(folio);
+
+	/*
+	 * Order reads for folio refcount and dirty flag
+	 * (see comments in __remove_mapping()).
+	 */
+	smp_rmb();
+
+	/*
+	 * If the PMD or folio is redirtied at this point, or if there are
+	 * unexpected references, we will give up to discard this folio
+	 * and remap it.
+	 *
+	 * The only folio refs must be one from isolation plus the rmap(s).
+	 */
+	if (ref_count != map_count + 1 || folio_test_dirty(folio) ||
+	    pmd_dirty(orig_pmd)) {
+		set_pmd_at(mm, addr, pmdp, orig_pmd);
+		return false;
+	}
+
+	folio_remove_rmap_pmd(folio, page, vma);
+	zap_deposited_table(mm, pmdp);
+	add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
+	folio_put(folio);
+
+	return true;
+}
+
+bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
+			   pmd_t *pmdp, struct folio *folio)
+{
+	VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
+	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
+	VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
+		return __discard_trans_pmd_locked(vma, addr, pmdp, folio);
+#endif
+
+	return false;
+}
+
 static void remap_page(struct folio *folio, unsigned long nr)
 {
 	int i = 0;
diff --git a/mm/rmap.c b/mm/rmap.c
index e42f436c7ff3..ab37af4f47aa 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1677,6 +1677,9 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		}
 
 		if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
+			if (unmap_huge_pmd_locked(vma, range.start, pvmw.pmd,
+						  folio))
+				goto walk_done;
 			/*
 			 * We temporarily have to drop the PTL and start once
 			 * again from that now-PTE-mapped page table.
-- 
2.33.1